Motor-driven farm vehicle gearboxes can provide a large number of possible speed ratios for the vehicle so that working equipment can be carried or pulled under optimum conditions considering the nature of the ground to be worked. In other words, gearboxes can adapt the vehicle speed to match encountered variations in resistance to progress. Furthermore, recent gearboxes include at least two parallel shaft clutches to make it possible to change from one gear to another without releasing the load, in other words without the loss of driving force during the gear change.
Normal farm vehicle gearboxes also include a large number of possible speed ratios with close speed ranges.
It is now required that gearboxes should not only have a large number of possible speed ratios, but also that speed ratios should vary over a range from very slow to very fast, for example with a range from a few hundred metres per hour up to 50 km per hour.
To achieve this, it has been thought that planetary gear systems can be staged inside gearboxes. Large reduction ratios can be obtained with this mechanical transmission system, although it is relatively compact compared with normal gearboxes.
Refer to document DE 10 2013 110 709 A1 that discloses a gearbox unit designed precisely to obtain wide speed ranges. It thus includes a system of staged planetary gears coupled at the input to the drive shaft and capable of providing a plurality of first transmission ratios. The output of the planetary gear system is coupled with two parallel clutch devices. Each clutch device comprises an input shaft clutch coupled to the planetary gear system and another output shaft clutch. For each clutch device, the unit comprises a large high speed clutch wheel and a small low speed clutch wheel both installed free to rotate on the output shaft clutch to be able to provide two second transmission ratios. It also includes a clutch coupling device for each clutch device, so that either the low speed or high speed wheels can be coupled with the output shaft clutch. The unit also comprises an output shaft comprising a small high speed output wheel engaging with the large high speed clutch wheels and a large low speed output wheel engaging with small low speed clutch wheels of two clutch devices. It is understood in this case that the wheels mentioned herein are gear wheels with parallel teeth. However, the output shaft is fitted with a conical pinion engaging a conical wheel that is mechanically coupled to a differential.
With this gearbox unit, a motor-driven vehicle is obtained with a large number of speed ratios over a wide speed range. However, premature wear of the clutch wheels has been observed for some vehicles, and under some working conditions. Furthermore, the large number of successive or parallel gear trains in such gearbox units makes their mechanical efficiency poor.
Another problem that arises and that this invention is intended to solve is to provide a gearbox unit that in particular can reduce wear of its component elements and also improve the mechanical efficiency.